This invention relates to a scanning optical device and, more particularly, a scanning optical device in which a light beam emitted from light source means is deflected by a deflecting element and, with the deflected light beam and through an imaging element having fxcex8 characteristic, a surface to be scanned is optically scanned, whereby imagewise information is recorded thereon. The scanning optical device of the present invention is particularly suitably usable in an image forming apparatus such as a laser beam printer having an electrophotographic process, a digital copying machine, or a multifunction printer, for example.
Conventionally, in scanning optical devices for a laser beam printer (LBP), for example, a light beam emitted from light source means is modulated in accordance with an imagewise signal. The modulated light beam is periodically deflected by means of an optical deflector which may comprise a polygonal mirror, for example. Then the deflected light is converged by an imaging optical system having fxcex8 characteristic, into a light spot on the surface of a recording medium having photosensitivity, to optically scan the same, whereby image recording is accomplished.
FIG. 18 is a schematic view of a conventional scanning optical device. In FIG. 18, a divergent light beam emitted from light source means 1 is transformed by a collimator lens 2 into substantially parallel light. After being restricted by a stop 3, the light beam enters a cylindrical lens 4. As regards the parallel light beam entered into the cylindrical lens 4, with respect to a major (primary) scan plane, the light is directly emitted as it is, whereas, with respect to a minor (secondary) scan plane, the light is converged. As a result, the light is imaged on a reflection surface of a light deflector (polygonal mirror) 5, as approximately a linear image. The light beam reflectively deflected by the reflection surface of the light deflector 5 is directed through a scanning optical element (fxcex8 lens) having fxcex8 characteristic to a surface 8 to be scanned. By rotating the light deflector 5 in a direction of an arrow, the surface 8 is scanned with light.
In such scanning optical device, for high-precision recording of imagewise information, there are requirements to be satisfied, as follows.
1) Curvature of field should be well corrected over the whole surface to be scanned.
2) Distortion characteristic (fxcex8 characteristic) with velocity uniformness should be maintained between the scan angle xcex8 and the image height Y.
3) Spot diameter on the image plane should be uniform, with respect to different image heights.
Many proposals have been made to provide a scanning optical device satisfying these optical characteristics or a correction optical system (scanning optical element) therefor.
On the other hand, there are similar requirements in relation to scanning optical devices, to meet reduction in size and cost of laser beam printers or digital copying machines. As an example of a structure satisfying such requirements, Japanese Laid-Open Patent Application, Laid-Open Nos. 4-50908 and 9-33850 propose a scanning optical system in which a scanning optical element is constituted by a single element.
In Japanese Laid-Open Patent Application, Laid-Open No. 4-50908, a high-order aspherical surface is used in a scanning optical element with respect to a major (primary) scan direction, to correct aberration characteristics relatively well. However, since the magnification between a light deflector and the surface to be scanned, with respect to the minor (secondary) scan direction, is not constant, the spot diameter in the minor scan direction may change with the image height.
On the other hand, Japanese Laid-Open Patent Application, Laid-Open No. 9-33850 shows a scanning optical device having a scanning optical element which is arranged so that, in at least two of the lens surfaces thereof, the curvature in the minor scan direction changes continuously within an effective portion of an imaging lens and in the major scan direction, independently from the curvature in the major scan direction. Thus, the position of a principal plane in the minor scan direction is controlled by bending two surfaces, to make constant the minor scan magnification at different image heights, thereby to make the spot diameter constant.
In this proposal, however, in order to make the minor scan magnification constant, at least two surfaces are bent so as to control the position of the principal plane to make the magnification constant. Thus, the shape in major direction and the shape in minor direction are determined completely independently of each other. Therefore, in order to keep the lens thickness small, in many cases, the lens shape in the major scan direction contains a relatively large aspherical amount.
With a lens having a large aspherical amount in the major scan direction, as described above, optical performance would be much degraded by an error of disposition of each lens surface and of the lens itself. As regards degradation of optical performance, particularly the curvature of scan line in minor scan direction leads to a serious problem because it can not be corrected by adjustment of a mirror or the like, disposed in the major assembly of the device, as contrasted to deviation of scan line height or scan line tilt. In consideration of it, in order to minimize the scan line curvature, each lens surface and the lens itself must be disposed very precisely, exactly in accordance with design values. Alternatively, an adjusting mechanism has to be added for the lens, to adjust the disposition exactly as designed.
As regards color image forming apparatuses in which four photosensitive members (photosensitive drums) each being provided with an optical scanning device are used and, while a latent image is produced by use of laser light, images of four colors of Y (yellow), M (magenta), C (cyan) and Bk (black) are formed on surfaces of corresponding photosensitive members, respectively, the images of four colors Y, M, C and Bk thus formed on the respective photosensitive members should be superposed one upon another on a transfer material such as a paper sheet. If, therefore, the scan lines of the scanning devices corresponding to the four photosensitive members, respectively, have a curvature, an error would be produced in the scan line shape of the four colors which would cause color misregistration in images formed on the transfer material. This results in considerable degradation of the image performance.
FIG. 19 illustrates a shift amount of scan line on the surface to be scanned, where, in the structure of an embodiment shown in the aforementioned Japanese patent application, a scanning optical element and optical surfaces thereof are deviated by 50 microns in a direction (minor scan direction) perpendicular to the major scan direction. It is seen from the drawing that a large scan line curvature is produced by deviation of an optical surface and that, in order to obtain a high-quality image, the disposition precision must be improved and deviation adjustment is required.
In FIG. 19, a curve R1 depicts the amount of scan line curvature where only a light entrance surface of a fxcex8 lens 6 is deviated in the minor scan direction. Curve R2 depicts the amount of scan line curvature where only a light exit surface of the fxcex8 lens 6 is deviated. A block curve depicts the amount of scan line curvature where both the light entrance surface and the light exit surface of the fxcex8 lens 6 are deviated.
It is a first object of the present invention to provide a scanning optical device and an image forming apparatus using the same, by which, in a molded lens, even if there is produced a deviation of a light exit surface in Z direction (minor scan direction), with respect to a light entrance surface, due to insufficient precision of molding or the like, since the minor scan magnification at each surface of a scanning lens can be held substantially constant, a curvature of scan line at the surface to be scanned due to the deviation in the minor scan direction, can be avoided.
It is a second object of the present invention to provide a scanning optical device and an image forming apparatus using the same, by which field-curvature characteristic, fxcex8 characteristic for constant-speed scan, and wavefront aberration which are characteristics of an optical system are additionally well corrected, and by which, through appropriate shape setting to a scanning optical element, a curvature of scan line due to an error in disposition of a molded lens can be minimized.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.